Formation of a multilayered coating film by applying a coating composition having uniformly dissolved or dispersed therein a part or all of main components onto a support has been conventionally achieved by a successive coating technique in which each layer is separately coated and dried to build up multiple layers, or by an advanced technique called successive multilayer coating (or a so-called wet on wet coating) or simultaneous multilayer coating which comprises forming a plurality of layers through one coating step, followed by drying all at once. For example, Japanese Patent Application (OPI) No. 189969/84 (the term "OPI" as used herein means "unexamined published Japanese patent application") discloses formation of a non-aqueous multilayered coating film.
However, according to the successive coating process, a coating composition applied on a dried coating film re-dissolves or swells a part of the dried coating film to cause interlaminar mixing, resulting in failure to fully attaining the desired performance. In order to avoid interlaminar mixing, an intermediate layer should be provided, which leads to increased costs for both materials and production. Particularly, this technique increases costs due to repetition of coating and drying steps.
Also, successive multilayer coating and simultaneous multilayer coating processes require delicate adjustment of mutual solubility between solvents and solutes of adjacent layers and relationships of adjacent layers in physical properties of the coating compositions. If good adjustments cannot be made, interlaminar mixing will occur before each layer is fixed, thus failing to obtain the expected performance. Hence, much study and effort have been required for putting these technique to practical use.